Testing head having cantilever probes

ABSTRACT

A testing head having cantilever probes is presented. The testing head, comprises a backing ring and a resin holder attached to the backing ring, as well as a plurality of contact probes held by the resin holder and formed with respective contact tips arranged to mechanically and electrically contact a plurality of contact pads of at least one device to be tested. The holder is formed with at least one suitably shaped outline to allow different probe rows to emerge in a cantilever manner.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a testing head having cantileverprobes, and more particularly to a testing head for use on semiconductorintegrated devices.

[0003] 2. Description of the Related Art

[0004] As is well known, a testing head is basically a device suitableto electrically interconnect a plurality of contact pads of amicrostructure and the corresponding channels of a testing machine thatis to perform the tests.

[0005] Integrated circuits are factory tested in order to spot andreject any circuits which are already defective during the productionphase. The testing heads are normally employed to electrically test theintegrated circuits “on wafer”, prior to cutting and mounting them in achip package.

[0006] As schematically shown in FIGS. 1 and 2, a testing head 1 havingcantilever probes usually comprises a backing ring 2, made of aluminumor ceramics, to which a resin holder 3 is attached, and that is suitableto hold a plurality of movable contact elements or probes 4, beingnormally wires made of special alloys having good electrical andmechanical properties, the probes being mounted to jut out of the resinholder 3 at plural points 5 and at a suitable angle from a plane β. Suchemerging probes are commonly known as “cantilever probes”.

[0007] In particular, each probe 4 has an end portion or contact tip 6,which is bent with an angle γ from the rest of the probe so that aplurality of contact pads 7 of a device to be tested is contacted. Thebent contact tips 6 are commonly referred to as the “crooks”.

[0008] The good connection of the probes 4 of the testing head 1 to thecontact pads 7 of a device to be tested is ensured by the testing head 1exerting a pressure on the device, whereby the probes 4 are verticallyflexed in the opposite direction from the device movement towards thetesting head 1.

[0009] As schematically shown in FIG. 3 for a single probe 4, as thedevice to be tested vertically moves against the contact tip 6, theprobe 4 flexes, and its elbow point X, situated at the transition fromthe contact tip 6 to a probe section 8 emerging from the resin holder 3,describes a circular arc.

[0010] Thus, the jutting probe section 8 forms a working arm of theprobe 4 adapted to flex vertically, and is commonly referred to as the“free length” of the probe.

[0011] The crooked shape of the probes 4 is designed to allow thecontact tips 6 of the probes 4 to skid, upon coming in touch with thecontact pads 7 of the device to be tested and during the pad overtravelbeyond a pre-set point of contact, across the contact pads 7 along adirection dictated by the arrangement geometry.

[0012] It should be noted that the force exerted on the contact pads 7by each probe 4 depends on many factors, among which are especially thetype of material forming the probe 4, the probe shape, the angle α madeby the probe contact tip 6, the length of the probe jutting section orfree length 8, and the amount of overtravel of the pads to be measured.These factors also determine the extent of the contact tips 6 skiddingon the contact pads 7, this being commonly known as the “scrub”.

[0013] It should be noted that, with a dense distribution of the contactpads 7, the probes 4 must be arranged in plural rows, and the lengthsLl, . . . Ln of the crooked ends vary accordingly, as schematicallyshown in FIG. 4.

[0014] Also known is to use backing rings 2, generally made of aluminumor ceramics, having different shapes depending on the set of contactpads 7 to be tested, so that the free lengths of the probes 4, and hencethe forces exerted by the latter to the contact pads 7, can be equalizedin the interest of even wear and performance of the testing head 1.

[0015] In particular, when the probes 4 are arranged in a plurality ofrows or levels, as schematically shown in FIGS. 5A, 5B and 5C, theemerging points 5 of the probes 4 from the resin 3, when viewedfrontally, make either a diagonal (FIG. 5A), or straight (FIG. 5B), orcombination pattern (FIG. 5C) that is dependent on constructionalrequirements.

[0016] The portions of the probes outside the backing ring 2 are usuallysoldered on a PC board 9, as shown in FIG. 1, to establish an electricalconnection between the testing head 1 having cantilever probes and thetesting machine.

[0017] It is therefore necessary that the outer portion of any probe 4can be recognized unfailingly in the probe bunch, so that it can besoldered on the PC board 9 in the correct manner.

[0018] In addition, the probes 4 extend with their sections outside thebacking ring 2 parallel to one another, as shown in FIG. 6A (side A),and the probes 4 for soldering on the PC board 9 are not easily singledout. It is also known the use of probes 4 with a radial spreading intheir portion outside the backing ring 2, as schematically shown in FIG.6A (side B).

[0019] The probes 4 can be arranged in a plurality of rows or layerssuch that they have a diagonal or a straight configuration, in eitherthe case of parallel or radial probes, as shown in FIG. 6A.

[0020]FIG. 6B shows, by way of example, an arrangement of the probes 4in three rows with a radial diagonal configuration, and FIG. 6C shows anarrangement of the probes 4 in three rows with a radial straightconfiguration.

[0021] It is, moreover, a known fact that certain electronic devices,e.g. memories, have contact pads disposed along two sides only.Accordingly, a number of such devices can be tested in parallel if theyare set in a single row.

[0022] A row of devices can be tested by suitably calibrating the insidedimensions of the backing ring 2.

[0023] When several rows of devices are to be tested in parallel—usuallytwo rows of eight devices or four rows of eight devices—multi-bridgebacking rings, schematically shown in FIGS. 7A and 7B, are used.

[0024] In particular, a multi-bridge backing ring 2 b includes aplurality of bridges 2 chaving a width dimension P inside the ring 2bperimeter, which bridges are suitable to carry probes for severaldevices to be tested in parallel. There are various techniques that canbe used in order to obtain the desired pressure uniformity on the probes4 against the corresponding contact pads 7.

[0025] A first known technique uses a multi-bridge backing ring 2 bhaving plural bridges 2 c inside its perimeter to define plural devicerows FILA1, FILA2, as schematically shown in FIGS. 7A and 7B.

[0026] The shape and dimensions of the multi-bridge backing ring 2 b andthe inner bridges 2 c are selected such that the jutting sections orfree lengths FL1, FL2, FL3, FL4, . . . of all the probes will beequalized. In this way, the probes 4 are all caused to abut on thecontact pads 7 with the same force.

[0027] A limitation comes to this prior technique from that the minimumwidth Lmin of a device to be tested cannot be less than the sum of theminimum length FLmin of the jutting section or minimum free length FL1,FL2, . . . of the probes 4 and the minimum theoretical width Pmin ofeach inner bridge 2 c, i.e.:

Lmin≧FLmin+Pmin,  (1)

[0028] as schematically shown in FIG. 7A.

[0029] A second prior technique uses probes of different types bound tothe same backing ring 2, as schematically shown in FIGS. 8A and 8B.

[0030] In particular, probes 4 of a larger diameter are used for theinnermost contact pads 7 within the backing ring perimeter, to haveequal forces exerted on the contact pads 7 even though the juttingsections or free lengths FL1, FL2, . . . may be different.

[0031] Using this technique, however, the dimensions and free lengths ofthe probes 4 are difficult to calibrate for an even pressure on all thecontact pads 7. In addition, the probes 4 which are to reach deviceslocated farther inwards than the probe emergence points on the backingring 2 will be those having the largest dimensions, as having thegreatest jutting sections or free lengths 8, thus enforcing reduceddensity for the contact pads 7 on the devices.

[0032] Also, neither of the above prior techniques would work where alarge number of devices are to be tested in parallel. In particular, thetesting heads so provided cannot test more than two rows of devices, andare definitely incapable of testing a matrix array of devices.

SUMMARY OF THE INVENTION

[0033] Embodiments of this invention provide testing heads formicrostructures, having a configuration which can facilitate theoperations of sorting out probes and soldering them on a PC board,specifically densely clustered probes, and allowing an unlimited numberof devices laid into plural rows to be tested.

[0034] One of the principles on which embodiments of the presentinvention stand is one of suitably shaping the resin holder attached tothe probe backing ring in order to provide a clearer view of each probein a cluster of probes during the soldering steps, while alsomaintaining an accurate control of the probe jutting sections or freelengths when a plurality of devices are under parallel test.

[0035] Presented is a testing head having cantilever probes andcomprising a backing ring and a resin holder attached to the backingring, as well as a plurality of contact probes held by the resin holderand formed with respective contact tips arranged to mechanically andelectrically contact a plurality of contact pads of at least one deviceto be tested, the holder being formed with at least one suitably shapedoutline to allow different probe rows to emerge in a cantilever manner.Additionally presented is a method of creating an electro/mechanicalconnection between a testing head and a test device.

[0036] The features and advantages of a testing head according to theinvention will become apparent from the following description ofembodiments thereof, given by way of non-limiting examples withreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0037]FIG. 1 is a top plan view of a testing head having cantileverprobes, according to an embodiment of the prior art;

[0038]FIG. 2 is a sectional view of the testing head having cantileverprobes according to the embodiment of FIG. 1;

[0039]FIG. 3 is a sectional view of a detail of the testing head havingcantilever probes according to the embodiment of FIG. 1;

[0040]FIG. 4 is a sectional view of a detail of a testing head havingcantilever probes according to a further embodiment of the prior art;

[0041]FIGS. 5A, 5B and 5C are schematic sectional views of a detail ofthe testing head having cantilever probes according to the embodiment ofFIG. 4;

[0042]FIGS. 6A, 6B and 6C are schematic views of a testing head havingcantilever probes according to a further embodiment of the prior art;

[0043]FIGS. 7A and 7B are schematic views of a testing head havingcantilever probes according to a further embodiment of the prior art;

[0044]FIGS. 8A and 8B are schematic views of a testing head havingcantilever probes according to a further embodiment of the prior art;

[0045]FIG. 9 is a schematic sectional view of a testing head havingcantilever probes according to an embodiment of the invention;

[0046]FIG. 10 is a schematic sectional view of a testing head havingcantilever probes according to the embodiment of FIG. 9; and

[0047]FIG. 11 is a schematic sectional view of the testing head havingcantilever probes according to a further embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0048] With reference to FIG. 9, a testing head 10 according to anembodiment of the invention, being employed to test at least one device,is shown. The testing head 10 comprises at least one backing ring 12having a resin holder 13 suitable to hold a plurality of probes 14.

[0049] The probes 14 are arranged in plural rows L1, L2, . . . , Ln inorder to increase their density, it being possible for the probes to belaid in a pattern of either diagonal or straight or combination lines,depending on individual requirements.

[0050] Advantageously according to a preferred embodiment of theinvention, the holder 13 has a suitably shaped outline 13 b incorrespondence of outer sections 19 of the probes 14, i.e. incorrespondence of a region of a PC board (not shown) where the probes 14are soldered on the board.

[0051] In particular, the outline 13 b shows a plurality of stepshelping the detection of specific probes within the bunch of probes thatemerge from the resin holder 13, by virtue of the probe selection havingbeen restricted to no more than one or a small number of probe rows.

[0052] In a preferred embodiment of the invention, the outline 13 bincludes a plurality of steps, all with the same width and height. Inparticular, the outline 13 b has as many steps as many the rows ofprobes 14 are.

[0053] According to an alternative embodiment the outline 13 b isprovided with a number of steps which is a submultiple of the number ofrows, with each step defining points of emergence of more than one rowof probes 14.

[0054] Assuming the probes 14 to be arranged in eight rows L1-L8, theoutline 13 b of the resin holder 13, as shown in FIG. 11, has foursteps, with each step defining the points of emergence of two rows ofprobes 14. This is given by way of non-limiting example of a testinghead according to an embodiment of the invention.

[0055] The number of rows per step may be varied, even within the sametesting head, to meet specific requirements. It should be noted that oneprobe row might include probes placed at different heights above theplane containing the plurality of devices 11 to be tested.

[0056] In this way, soldering an unlimited number of probes, indifferent rows, can be made easier by providing the outline 13 b with anappropriate number of steps.

[0057] Advantageously according to embodiments of the invention, theresin holder 13 has a larger size than the backing ring 12, e.g. byadding final filling resin 20 to blend these elements to each other, asschematically shown in FIG. 10.

[0058] Shown in FIG. 11 is testing head 10 according to a furtherembodiment of the invention. The testing head 10, for application to aplurality of devices 11 to be tested, comprises at least one backingring 12 having a resin holder 13 for holding a plurality of probes 14.

[0059] In particular, the probes 14 have an end portion or contact tip16 that is bent through a suitable angle from the probes to contact aplurality of contact pads 17 on said plurality of devices 11 to betested.

[0060] Furthermore, the probes 14 are arranged in several rows L1, L2, .. . , Ln for increasing their density, the probe layout in front viewbeing a pattern of diagonal or straight or combination lines, dependingon individual requirements.

[0061] Good contacting of the contact pads 17 of the device to be testedby the probes 14 of the testing head 10 is conventionally ensured by thepressure of the testing head 10 on the device, the probes 14 verticallyflexing in the opposite direction from the device direction of advancetoward the testing head 10.

[0062] Advantageously according to embodiments of the invention, theholder 13 has a suitably shaped outline 13 a in correspondence of thejutting sections or free lengths 18 of the probes 14, i.e. incorrespondence of the devices to be tested.

[0063] In particular, the outline 13 a shows a plurality of steps havingshape and dimensions suitably selected to ensure an even pressure of theprobes 14 on the contact pads 17 as the devices to be tested are broughtto contact the testing head 10.

[0064] In a preferred embodiment of the testing head 10, schematicallyshown in FIG. 11, the holder 13 has a saw-tooth outline 13 a,establishing a common level of emergence for said probes 14. In otherwords, the points 15 of emergence of the probes 14 from the resin holder13 lie on a parallel plane to the plane containing the plurality ofdevices 11 to be tested, but are shifted in position from one another inorder to reach contact pads 17 on different devices.

[0065] Advantageously according to a preferred embodiment of theinvention, the probes 14 have all jutting sections or free lengths 18that jut equally out of the emergence points 15. Thus, a plurality ofdevices 11 laid in several parallel rows can be tested by using equallydimensioned probes formed out of the same materials, at the same time asthe proper pressure can be maintained on the contact pads 17 of deviceslying in different rows, as schematically shown in FIG. 11.

[0066] It can be seen in FIG. 11 that the probes in touch with padslocated in adjacent device rows spaced a predetermined distance apart,known as the “scribing line”, belong to different rows but to the samestep. In this case, a variation in the jutting sections or free lengths18 must be provided. However, this variation is far less than inconventional testing heads, and can be readily compensated by alteringthe probe dimensions, e.g. varying the shape of the probe tip.

[0067] In addition, the testing heads 10 according to embodiments of theinvention obviate the shortcomings of conventional testing heads havingcantilever probes, and allow a matrix array of devices laid serially inplural rows to be tested.

[0068] In particular, the density of those probes 14 which are to testdevices placed farther inwards of the backing ring 12 is the same as thedensity of the probes 14 used for testing devices placed next to thebacking ring 12, thus providing for an equal density of contact pads 17on the devices to be tested, regardless of their positions in a matrixarray.

[0069] To summarize, the testing heads 10 according to embodiments ofthe invention allow probes 14 of equal size and formed out of the samematerial to be used for testing a matrix array of devices having anunlimited number of rows and columns.

[0070] Changes can be made to the invention in light of the abovedetailed description. In general, in the following claims, the termsused should not be construed to limit the invention to the specificembodiments disclosed in the specification and the claims, but should beconstrued to include all methods and devices that are in accordance withthe claims. Accordingly, the invention is not limited by the disclosure,but instead its scope is to be determined by the following claims.

1. Testing head having cantilever probes and comprising: a backing ringa holder attached to the backing ring; and a plurality of contact probesheld by the holder and formed with respective contact tips arranged tomechanically and electrically contact a plurality of contact pads of atleast one device to be tested, wherein the holder is formed with atleast one suitably shaped outline to allow different probe rows toemerge in a cantilever manner.
 2. The testing head of claim 1, whereinsaid shaped outline of the holder includes at least two steps formed toallow at least two different rows of probes to emerge.
 3. The testinghead of claim 1, wherein said shaped outline is provided incorrespondence of an outer section for soldering said probes.
 4. Thetesting head of claim 3, wherein said shaped outline of the holderincludes at least two steps formed to allow at least two different rowsof probes to emerge.
 5. The testing head of claim 4, wherein each stepof the shaped outline holds probes in a same row.
 6. The testing head ofclaim 4, wherein each step of the shaped outline holds probes indifferent rows.
 7. The testing head of claim 1, wherein said shapedoutline of the holder is positioned in correspondence of said device tobe tested.
 8. The testing head of claim 7, wherein said shaped outlineof the holder is a saw-tooth outline, whereby the emerging points ofsaid probes from said holder lie on a parallel plane to the planecontaining said device to be tested.
 9. The testing head of claim 8,wherein said shaped outline produces jutting sections of said probesfrom the holder that are substantially identical with one another. 10.The testing head of claim 1, wherein the holder is formed with anothershaped outline in correspondence of said device to be tested.
 11. Thetesting head of claim 10, wherein said shaped outline of the holderincludes at least two steps formed to allow at least two different proberows to emerge, and said another shaped outline of the holder is asaw-tooth outline, whereby the emerging points of said probes from saidholder lie on a parallel plane to the plane containing said device to betested.
 12. The testing head of claims 1, further comprising a pluralityof probes arranged in rows to test a matrix array of devices seriallylaid in rows, each device in said array being reached by probes ofdifferent rows.
 13. The testing head of claim 1, wherein said holder islarger than said backing ring.
 14. The testing head of claim 1, whereinsaid holder is a resin holder.
 15. The testing head of claim 1, furthercomprising a finishing filler resin to blend together said holder andsaid backing ring.
 16. Testing head having cantilever probes andcomprising: a holder; and a plurality of contact probes held by theholder and formed with respective contact tips arranged to mechanicallyand electrically contact a plurality of contact pads of at least onedevice to be tested, wherein the holder is structured to held saidcontact probes to form a plurality of probe rows and to allow differentprobe rows to emerge in a cantilever manner.
 17. The testing head ofclaim 16, wherein said holder is formed with at least one suitablyshaped outline.
 18. The testing head of claim 17, wherein said shapedoutline includes at least two steps formed to allow at least twodifferent rows of probes to emerge.
 19. The testing head of claim 17,wherein said shaped outline is provided in correspondence of an outersection for soldering said probes.
 20. The testing head of claim 18,wherein each step of the shaped outline holds probes in a same row. 21.The testing head of claim 18, wherein each step of the shaped outlineholds probes in different rows.
 22. The testing head of claim 17,wherein said shaped outline of the holder is positioned incorrespondence of said device to be tested.
 23. The testing head ofclaim 17, wherein said shaped outline is a saw-tooth outline, wherebythe emerging points of said probes from said holder lie on a parallelplane to the plane containing said device to be tested.
 24. The testinghead of claim 17, wherein said shaped outline produces jutting sectionsof said probes from the holder that are substantially identical with oneanother.
 25. The testing head of claim 17, wherein the holder is formedwith another shaped outline in correspondence of said device to betested.
 26. The testing head of claim 25, wherein said shaped outlineincludes at least two steps formed to allow at least two different proberows to emerge, and said another shaped outline is a saw-tooth outline,whereby the emerging points of said probes from said holder lie on aparallel plane to the plane containing said device to be tested.
 27. Thetesting head of claim 16, further comprising a plurality of probesarranged in rows to test a matrix array of devices serially laid inrows, each device in said array being reached by probes of differentrows.
 28. The testing head of claim 16, wherein said holder is attachedto a backing ring.
 29. The testing head of claim 28, wherein said holderis a resin holder.
 30. The testing head of claim 28, wherein said holderis larger than said backing ring.
 31. The testing head of claim 28,further comprising a finishing filler resin to blend together saidholder and said backing ring.
 32. A method for creating anelectro/mechanical connection between a testing head and a test device,the method comprising: and holding a plurality of contact probes in thetesting head in a cantilever manner, the contact probes having contacttips bent through suitable angles from the contact probes to contact aplurality of contact pads on a test device; establishing a common levelof emergence for the contact probes from an holder of the testing head,the points of emergence of the contact probes lying on a parallel planeto the plane of the test device, and being shifted in position from oneanother in order to reach different contact pads; causing the contactingtips of the contact probes to abut against the contact pads as the testdevice is pressed against the contacting tips; causing the contactprobes to vertically flexing in the opposite direction from the devicedirection of advance toward the testing head.
 33. The method of claim32, wherein establishing a common level of emergence for the contactprobes from an holder of the testing head comprises providing an holderformed with at least one suitably shaped outline to allow differentprobe rows to emerge in a cantilever manner.
 34. The method of claim 33,wherein said shaped outline of the holder includes at least two stepsformed to allow at least two different rows of probes to emerge.
 35. Themethod of claim 33, wherein said shaped outline is provided incorrespondence of an outer section for soldering said probes.
 36. Themethod of claim 33, wherein said shaped outline of the holder includesat least two steps formed to allow at least two different rows of probesto emerge.
 37. The method of claim 36, wherein each step of the shapedoutline holds probes in a same row.
 38. The method of claim 36, whereineach step of the shaped outline holds probes in different rows.
 39. Themethod of claim 33, wherein said shaped outline of the holder ispositioned in correspondence of said test device.
 40. The method ofclaim 39, wherein said shaped outline of the holder is a saw-toothoutline, whereby the emerging points of said probes from said holder lieon a parallel plane to the plane containing said test device.
 41. Themethod of claim 40, wherein said shaped outline produces juttingsections of said probes from the holder that are substantially identicalwith one another.
 42. The method of claim 33, wherein the holder isformed with another shaped outline in correspondence of said testdevice.
 43. The method of claim 42, wherein said shaped outline of theholder includes at least two steps formed to allow at least twodifferent probe rows to emerge, and said another shaped outline of theholder is a saw-tooth outline, whereby the emerging points of saidprobes from said holder lie on a parallel plane to the plane containingsaid test device.
 44. The method of claim 32, further comprising, beforethe contact pad touches the contacting tip: blending together saidholder and a backing ring by means of a finishing filler resin.
 45. Amethod for creating an electro/mechanical connection between a testinghead and a plurality of test devices, the method comprising: holding aplurality of contact probes in the testing head in a cantilever manner,the contact probes having contact tips bent through suitable angles fromthe contact probes to contact a plurality of contact pads on the planecontaining the plurality of test devices; establishing a common level ofemergence for the contact probes from an holder of the testing head, thepoints of emergence of the contact probes lying on a parallel plane tothe plane containing the plurality of test devices, and being shifted inposition from one another in order to reach contact pads of differenttest devices; causing the contacting tips of the contact probes to abutagainst the contact pads as the plurality of test devices is pressedagainst the contacting tips; and causing the contact probes tovertically flexing in the opposite direction from the devices directionof advance toward the testing head.
 46. The method of claim 45, whereinsaid plurality of probes is arranged in rows to test a matrix array ofdevices serially laid in rows, each device in said array being reachedby probes of different rows.
 47. The method of claim 45, whereinestablishing a common level of emergence for the contact probes from anholder of the testing head comprises providing an holder formed with atleast one suitably shaped outline to allow different probe rows toemerge in a cantilever manner.
 48. The method of claim 47, wherein saidshaped outline of the holder includes at least two steps formed to allowat least two different rows of probes to emerge.
 49. The method of claim47, wherein said shaped outline is provided in correspondence of anouter section for soldering said probes.
 50. The method of claim 47,wherein said shaped outline of the holder includes at least two stepsformed to allow at least two different rows of probes to emerge.
 51. Themethod of claim 50, wherein each step of the shaped outline holds probesin a same row.
 52. The method of claim 50, wherein each step of theshaped outline holds probes in different rows.
 53. The method of claim47, wherein said shaped outline of the holder is positioned incorrespondence of said plurality of test devices.
 54. The method ofclaim 53, wherein said shaped outline of the holder is a saw-toothoutline, whereby the emerging points of said probes from said holder lieon a parallel plane to the plane containing said plurality of testdevices.
 55. The method of claim 54, wherein said shaped outlineproduces jutting sections of said probes from the holder that aresubstantially identical with one another.
 56. The method of claim 47,wherein the holder is formed with another shaped outline incorrespondence of said plurality of test devices.
 57. The method ofclaim 56, wherein said shaped outline of the holder includes at leasttwo steps formed to allow at least two different probe rows to emerge,and said another shaped outline of the holder is a saw-tooth outline,whereby the emerging points of said probes from said holder lie on aparallel plane to the plane containing said plurality of test devices.58. The method of claim 45, further comprising, before the contact padtouches the contacting tip: blending together said holder and a backingring by means of a finishing filler resin.